1. Field of the Invention
The present invention relates to a method for manufacturing an electronic device including a crystalline material, and particularly to a method for manufacturing a thin film piezoelectric single crystal device.
2. Description of the Related Art
Various electronic devices, such as semiconductors and piezoelectric bodies, that include a crystalline material having certain electrical characteristics are currently manufactured and used. Among the various electronic devices, a surface acoustic wave (SAW) device, a film bulk acoustic resonator (F-BAR) device, and other devices are exemplified as a piezoelectric device. Furthermore, many thin film piezoelectric devices obtained by reducing the thickness of a piezoelectric body have been developed as the piezoelectric device because they support high-frequency Lamb wave devices. A method for manufacturing a piezoelectric thin film for forming such a thin film piezoelectric device is disclosed, for example, in Y. Osugi et al.: “Single Crystal FBAR With LiNO3 and LiTaO3”, 2007 IEEE MTT-S International Microwave Symposium, pp. 873-876 and Japanese Unexamined Patent Application Publication No. 2002-534886.
The above-mentioned article, “Single Crystal FBAR With LiNO3 and LiTaO3”, discloses a deposition method in which a material defining a piezoelectric single crystal, such as AlN or ZnO, is deposited by sputtering or chemical-vapor deposition (CVD) and a polishing method in which a piezoelectric single crystal substrate having a desired thickness is bonded to a supporting substrate and then thinned by polishing.
Japanese Unexamined Patent Application Publication No. 2002-534886 discloses a method called a smart cut method. In the smart cut method, for example, a LiNO3 substrate or a LiTaO3 substrate, particularly a piezoelectric substrate composed of such a material is hereinafter referred to as an LN substrate or an LT substrate, is used as a piezoelectric material and the subsequent processes are performed: (1) an ion implantation layer is formed at a desired depth, where a piezoelectric thin film is to be formed, from the surface of a piezoelectric substrate by implanting ions into the piezoelectric substrate under suitable conditions; and (2) the piezoelectric substrate in which the ion implantation layer has been formed is heated. Through the heating treatment, the piezoelectric thin film is obtained from the piezoelectric substrate using the ion implantation layer as a detaching interface.
However, when the deposition method described above is used, the material is limited to AlN or other suitable material because of, for instance, the film formation temperature or the film formation conditions for obtaining a desired oriented film. Furthermore, since the orientation direction of a crystal axis cannot be controlled, it is difficult to form a crystal in consideration of vibration mode.
When the polishing method described above is used, since the portion other than the piezoelectric thin film is treated as polishing waste, the piezoelectric material is not efficiently used. In addition, it is difficult to form a piezoelectric thin film having a uniform thickness due to polishing conditions, variations in the shape of the original piezoelectric substrate, or other factors, which decreases productivity.
When the smart cut method described above is used, heating is performed in the detachment step. However, since the heating needs to be performed at a high temperature of about 450° C., the following problems arise.
Since the Curie temperature of the piezoelectric substrate is less than the heating temperature, the piezoelectric substrate is heated to a temperature greater than its Curie temperature. As a result, its piezoelectricity deteriorates, and the piezoelectric characteristics of the piezoelectric thin film after detachment are deteriorated.
When an LN substrate or an LT substrate is used as the piezoelectric substrate, oxygen in the piezoelectric substrate diffuses to the outside due to high-temperature heating. Thus, the insulation of the piezoelectric thin film obtained by performing detachment of the piezoelectric substrate near its surface is degraded, and its piezoelectric characteristics are deteriorated.
When an electrode is formed on the surface of the piezoelectric thin film side of the piezoelectric substrate, a metallic element defining the electrode diffuses into the piezoelectric thin film due to high-temperature heating. Consequently, the crystal structure of the piezoelectric thin film is likely to be broken and transmission loss in the piezoelectric thin film is increased.
In the structure in which the piezoelectric thin film is supported by a supporting substrate, the coefficients of linear expansion between the piezoelectric thin film and the supporting substrate need to be as close as possible to each other. Otherwise, breakage is likely to occur at the joint between the piezoelectric thin film and the supporting substrate due to the difference in coefficients of linear expansion during high-temperature heating. This severely limits the materials which can be used for the supporting substrate.
Since the temperature cannot be rapidly increased to the end-point temperature and then rapidly decreased during heating treatment, the higher heating temperature results in longer processing times.